Mass flow control for microbial fuel cells and microbial electrolysis cells for hydrogen production
Fuel cell and electrolysis research are both critical to the growth of the future hydrogen economy in which hydrogen will play the central role as the dominant fuel relative to hydrocarbons. To achieve this objective, hydrogen fuel cell and electrolysis researchers are focused on achieving goals such as:
- Lowering fixed costs of fuel cells and electrolysis cells
- Lowering general energy requirements
- Increasing energy output in fuel cells
- Increasing hydrogen production in electrolysis cells
- Optimizing fuel cell and electrolysis cell operating conditions (examples: temperature, pressure, flow rate, etc.)
What are microbial fuel cells and microbial electrolysis cells?
Two emerging bioelectrochemical systems (BESs) with exciting possibilities for solving some of the above challenges include microbial fuel cells and microbial electrolysis cells.
Microbial fuel cells
Invented in the mid 2000s, microbial fuel cells (MFCs) describe fuel cells which generate bioelectricity by regulating biochemical reactions catalyzed by using exoelectrogenic bacteria, or bacteria with the capability to convert biochemical energy to bioelectrical energy. These microbial fuel cells can be used for wastewater treatment, renewable energy production, water reuse, and bioremediation.
In this fuel cell type, bacteria generate electrons and protons in the anode chamber during the oxidation of organic matter, such as acetate. Electrons travel through an external electric circuit to terminal electron acceptors (such as oxygen, nitrate, etc.) located in the cathode chamber, undergoing reduction reactions. Simultaneously, protons and cations travel to the cathode chamber through either a membrane or through an electrolyte.
Microbial electrolysis cells for hydrogen production
Conversely, microbial electrolysis cells (MECs) generate hydrogen gas (or other important gases) by using exoelectrogenic bacteria combined with a small external voltage to drive electrolysis. Since the operating power requirements are relatively small, the extra voltage can be easily supplied using solar energy. This allows for classification as a renewable form of hydrogen production.
By combining both techniques, a microbial fuel cell can be used to generate electricity to power a microbial electrolysis cell. This enables the passive generation of hydrogen gas from biofluids using only renewable and sustainable energy sources.
Optimizing microbial fuel cells
Microbial fuel cells can be designed in either single or double chamber configurations.
In single chamber microbial fuel cells, the chamber includes an air cathode (in which oxygen flows from micro orifices from ambient air) connected to a membrane and an anode chamber.
Double-chamber microbial fuel cells are divided into a cathode chamber and an anode chamber through a proton exchange membrane. Chemical oxygen demand (COD) biomass solutions run through the anode chambers, releasing electrons and protons after interaction with bacteria located on the anode to generate electricity.
Liquid flow controllers regulate the flow of biomass solutions into the chamber/s in both configurations. Depending on the design, either gas or liquid mass flow controllers are used to flow oxygen solution into the double-chamber configuration.
Alicat’s liquid flow controllers such as LC-Series or CODA KC-Series can regulate and optimize the flow of biomass solutions into and out of the fuel cell. Adjusting flow rates allows for researchers to determine the ideal flow operating conditions.
Additional features and specifications for these products include:
- LC-Series NIST-traceable accuracy up to ±2% of full scale
- CODA KC-Series Coriolis controllers NIST-traceable liquid accuracy up to ±0.2% of reading or ±0.05% of full scale, whichever is greater
- LC-Series repeatability to ±0.2% of full scale
- CODA KC-Series repeatability to ±0.05% of reading or ±0.025% of full scale, whichever is greater
- Analog, serial, and industrial protocol communication options for scheduled commands
Optimizing microbial hydrogen electrolysis cells
Microbial electrolysis cells generate hydrogen or other important products of electrolysis by using microbes and external voltage to drive biochemical reactions.
Although microbial electrolysis cells are relatively new, there are several different designs which have been implemented so far. Some of these include:
- Continuous flow with effluent recycling
- Continuous flow without effluent recycling
Fed-batch microbial hydrogen electrolysis cells
In fed-batch systems, a COD biomass solution is controlled by a liquid mass flow controller and flows into a chamber with a series of stacked MECs. This reacts for a number of days and is then purged out and is replaced by another batch of biomass solution. This process is repeated.
Alicat’s liquid mass flow controllers can communicate to totalize and to control batching using various serial, analog, or industrial protocol options. This allows for automation of fed-batch systems.
Continuous flow with effluent recycling microbial hydrogen electrolysis cells
In continuous flow with effluent recycling, a biomass solution is continuously pumped between a reservoir and the reaction chamber at a low flow rate. The solution reacts with the microbial catalysts to produce hydrogen or other important gases, cycling through the system multiple times.
Alicat’s CODA KC-Series can be used with a meter-pump for liquid flow control without need for an external pressure source. This combination allows for movement and control of flow throughout the system, optimizing performance.
Continuous flow without effluent recycling microbial hydrogen electrolysis cells
In continuous flow without effluent recycling, COD is continuously pumped with new biomass solution. The effluent is outflowed as opposed to being recycled through the system.
As in continuous flow with effluent recycling, a KC-Series liquid controller can be combined with a meter-pump for flow regulation.
In summary, Alicat liquid mass flow controllers such as LC-Series and KC-Series add value to these systems by:
- Increasing the accuracy, repeatability, and precision of flow measurements
- Providing batching and totalizing options for automating batches and switching biofluids
- KC-Series compatibility with changing or unknown fluid composition